1. Field of Invention
This invention relates to an NMR (Nuclear Magnetic Resonance) image distortion correction method, and more particularly to such a method for correcting distortion of a reconstructed image caused by non-uniform static magnetic field.
2. Discussion of the Prior Art
An NMR imaging apparatus includes a magnetic unit having a static magnetic field coil for generating an uniform static magnetic field and a gradient magnetic field coil for generating a magnetic field whose direction coincides with the direction of the static magnetic field and which has a linearly extending gradient in each of the x, y and z directions. The NMR imaging apparatus further includes a transmitter/receiver unit for applying high frequency pulses (i.e. high frequency electromagnetic waves) to a body being examined, which body is disposed in the magnetic field generated by the magnetic unit, and for detecting an NMR signal from the body being examined; and a control/image processing unit for controlling operations of both the transmitter/receiver unit and the magnetic unit and for reconstructing the image by processing the detection data. In such an NMR imaging apparatus, a static magnetic field of a predetermined strength is formed by the static magnetic field in a predefined space in which the body being examined is placed, and an excitation pulse sequence and a gradient magnetic field sequence, according to the Fourier method, for example, are executed under control of the control/image processing unit to thereby measure the NMR signal. Based on the resulting data, an image of a cross-section of the body being examined is the reconstructed.
In this case, should the static magnetic field be non-uniform, the reconstruction image contains distortions, such as positional distortion and density distortion. In order to prevent such distortions, it is necessary to use a magnetic unit which generates a highly uniform static magnetic field. However, such a magnetic unit is expensive. Furthermore, the location of such a unit is restricted to a place where the amount of magnetic material is limited.
Furthermore, although it is desired that the lowest possible reading gradient be used so as to improve the Signal to Noise (S/N) ratio of the image, the distortion of the image increases as the reading gradient is reduced. Thus, under present circumstances, the reading gradient cannot be lowered to the limit determined by the echo signal time and the spatial resolution. However, despite the presence of such non-uniform static magnetic field, methods have been proposed for reducing image distortion, although the prior art methods have all been unsatisfactory.
In one conventional method, the image distortion is corrected in such a manner that the non-uniformity of the static magnetic field in three-dimensional space is measured in advance and then the image distortion is corrected by using data obtained upon measurement. (see IEEE Transaction on Medical Imaging, Vo. M1-4, No. 4, Dec. 1986, pp 193 to 199).
In another conventional method, with an expression representing the static magnetic field as a function of the three-dimensional space, the non-uniformity of the static magnetic field in a desired scan plane is computed and then the image distortion is corrected using the computed value.
The former method is not practical in view of the fact that the amount of data becomes too voluminous if the distribution data representative of the non-uniformity in the three-dimensional static magnetic field is given as being the same size as the matrix of the image reconstruction (e.g. 128.sup.3 to 256.sup.3).
On the other hand, the latter method is not suitable for high speed on-line image correction, since a large amount of computation is involved due to the inclusion in the expression of high order terms representing the non-uniformity of static magnetic field which cannot be corrected with a shim coil.